The top of the right side wall or the left side wall of the existing toilet tank is assembled with a control mechanism for the flapper drain valve of the bar shaped tank, which comprising a screw, a bolt, a lever and a lever handle with a revolving shaft, the revolving shaft is sleeved to the bolt, the end of the revolving shaft is connected to a lever of curved, the bolt is fixedly connected to the wall of the tank by nut. By driving the lever handle revolved, the revolving shaft drives the copper lever revolved. The flapper of the existing drain valve is connected to the end of the copper lever by chain, the end of the copper lever is moved up or down with revolving. The flapper is open or closed by pulling the chain or releasing the chain. As the copper lever is curved in a certain angle to realize the motion of the lever body in revolving, the motion trace of the copper lever is not a vertical surface but a curved surface in vertical direction when the copper lever is raised. A larger distance between the copper lever and the front wall of the tank is needed for the revolving room, so a larger width of the tank is needed. The lever control mechanism is inapplicable in the tank with small width.